1. Field of the Invention
The present invention relates to a circuit board, particularly a flexible printed circuit board, a connection structure using the circuit board, and an apparatus having the connection structure.
2. Description of the Related Art
Unlike a rigid board, a flexible printed circuit (FPC) board can be bent or folded, and therefore is distinctive in its high degree of freedom in arrangement. Thus, the FPC board is useful for an apparatus that has a small space available for wiring arrangement such as a flat panel display typified by a liquid crystal display panel.
One end of the FPC board is connected to a connector provided in an external circuit board or the like. See, e.g., JP 2004-178959 A (FIGS. 1 to 4) (Patent Document 1) and JP 7 (1995)-288371 A (FIGS. 1, 3, 9, 13 and 19) (Patent Document 2). Moreover, due to the recent development in mounting technology, electronic components can be mounted on the FPC board. This can make the apparatus thinner or lighter. Hereinafter, a conventional connection structure of the FPC board and the connector will be described with reference to the drawings.
FIGS. 5A and 5B are cross-sectional views showing an example of a FPC board and a connection structure in a conventional technique. FIG. 5A is a cross-sectional view of the connection structure. FIG. 5B is a cross-sectional view of the FPC board taken along the line B-B′ in FIG. 5A. In FIGS. 5A and 5B, the cross section of a conductive member is not hatched, and electronic components are shown only in outline.
As shown in FIG. 5A, a FPC board 30 is connected to a connector 40, thereby forming a connection structure. In FIGS. 5A and 5B, the FPC board 30 includes a base substrate 31 with flexibility, a signal wiring layer 32a provided on one surface of the base substrate 31, and a signal wiring layer 32b provided on the other surface of the base substrate 31. The signal wiring layers 32a, 32b have signal conductors, respectively.
Electronic components 39a are mounted on one surface of the FPC board 30. Moreover, to improve the packaging density, electronic components 39b also are mounted on the other surface of the FPC board 30. In the example of FIGS. 5A and 5B, the electronic components 39a, 39b are bump-mounted. Specifically, each of the electronic components 39a is connected electrically to the signal wiring layer 32a via bumps 37a and electrodes 36a provided on the signal wiring layer 32a. Each of the electronic components 39b is connected electrically to the signal wiring layer 32b via bumps 37b and electrodes 36b provided on the signal wiring layer 32b. 
The signal wiring layer 32a is covered with a protective film 34, and the signal wiring layer 32b is covered with a protective film 35. The protective films 34, 35 are made of a resin material. However, part of the signal conductors of the signal wiring layer 32a is not covered with the protective film 34, but exposed. There are a plurality of exposed portions 38, and they serve as terminals in the form of stripes.
Some of the signal conductors that are exposed partially from the protective film 34 are connected to the signal conductors of the signal wiring layer 32b via through holes 33 provided in the base substrate 31. Thus, in the FPC board 30, the through holes 33 are used to improve the packaging density by mounting the electronic components on both surfaces, while the terminals are concentrated on one surface.
The connector 40 is generally called a dual contact connector and includes a terminal member 41 and a housing 44 made of resin. The terminal member 41 has a U-shaped portion to fasten the FPC board 30 in the vertical direction. One end of the U-shaped portion of the terminal member 41 is a contact portion 42 that comes into contact with the upper surface of the FPC board 30, and the other end of the U-shaped portion is a contact portion 43 that comes into contact with the lower surface of the FPC board 30.
Accordingly, the connector 40 includes two upper and lower contact portions 42, 43, and thus can be used not only as an upper contact type connector, but also as a lower contact type connector. In the example of FIG. 5, the electric connection between the FPC board 30 and the connector 40 is made by connecting the upper contact portion 42 to the portion 38 (terminal) exposed from the protective film 34.
In another example of a connection structure using a FPC board, the FPC board is provided with a shielding layer to suppress the generation of noise due to the FPC board. This example will be described by referring to FIG. 6.
FIGS. 6A and 6B are cross-sectional views showing another example of a FPC board and a connection structure in a conventional technique. FIG. 6A is a cross-sectional view of the connection structure. FIG. 6B is a cross-sectional view of the FPC board taken along the line C-C′ in FIG. 6A. In FIGS. 6A and 6B, the cross section of a conductive member is not hatched, and electronic components are shown only in outline.
As shown in FIG. 6A, a FPC board 50 is connected to a connector 61, thereby forming a connection structure. However, as shown in FIGS. 6A and 6B, the FPC board 50 includes a base substrate 51 with flexibility, a signal wiring layer 52 provided on one surface of the base substrate 51, and a shielding layer 53 provided on the other surface of the base substrate 51.
The signal wiring layer 52 has signal conductors. Part of the signal conductors of the signal wiring layer 52 is not covered with a protective film 54, but exposed. There are a plurality of exposed portions 60, and they serve as terminals in the form of stripes. Electronic components 58 are bump-mounted on the signal wiring layer 52, and each of the electronic components 58 is connected electrically to the signal wiring layer 52 via bumps 59 and electrodes 57 provided on the signal wiring layer 52.
As shown in FIG. 6B, the shielding layer 53 is a metal layer that is formed over the entire surface on the other side of the base substrate 51 (see Patent Document 2). The shielding layer 53 is connected to the GND conductors of the signal wiring layer 52 via through holes 56. Moreover, the shielding layer 53 is covered with a protective film 55, while a portion of the shielding layer 53 remains exposed.
The connector 61 includes a terminal member 62 connected to the signal wiring layer 52, a terminal member 63 connected to the shielding layer 53, and a housing 64 for supporting the terminal members 62, 63. The connector 61 is formed so that the end of the FPC board 50 is fastened between the terminal members 62, 63. Moreover, the terminal member 63 is connected to a ground potential to suppress noise caused by the shielding layer.
In the connection structure of FIGS. 5A and 5B, the through holes are formed in the FPC board to improve the packaging density. Therefore, the wiring structure of the FPC board becomes complicated, and the manufacturing cost is high. Moreover, there is a limit to enlargement of the mounting area because of the formation of the through holes.
In the connection structure of FIGS. 6A and 6B, the shielding layer needs to be formed over the entire surface on one side of the FPC board. Therefore, the packaging density cannot be improved.